Plastic scintillator with effective pulse shape discrimination for neutron and gamma detection

ABSTRACT

In one embodiment, a scintillator material includes a polymer matrix; and a primary dye in the polymer matrix, the primary dye being a fluorescent dye, the primary dye being present in an amount of 5 wt % or more; wherein the scintillator material exhibits an optical response signature for neutrons that is different than an optical response signature for gamma rays. In another embodiment, a scintillator material includes a polymer matrix comprising at least one of: polyvinyl xylene (PVX); polyvinyl diphenyl; and polyvinyl tetrahydronaphthalene; and a primary dye in the polymer matrix, the primary dye being a fluorescent dye, the primary dye being present in an amount greater than 10 wt %. A total loading of dye in the scintillator material is sufficient to cause the scintillator material to exhibit a pulse-shape discrimination (PSD) figure of merit (FOM) of about at least 2.0.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 13/437,836, filed Apr. 2, 2012 and entitled “Plastic Scintillator with Effective Pulse Shape Discrimination for Neutron and Gamma Detection,” which claims priority to provisional U.S. Patent Application No. 61/476,071, filed on Apr. 15, 2011, to each of which priority is claimed and each of which is herein incorporated by reference.

The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC for the operation of Lawrence Livermore National Laboratory.

FIELD OF THE INVENTION

The present invention relates to spectroscopy, and more particular spectroscopy materials, systems and methods.

BACKGROUND

Radioactive materials are often detected and identified by measuring gamma-rays and/or neutrons emitted from the materials. The energy of gamma-rays is specific to that particular material and acts as a “finger print” to identify the material. Similarly, neutron energy is particular to the material, and may be used to identify the material. Of very high value are detectors capable of identifying the distinctive time-correlated signatures corresponding to neutrons and gammas emitted by fissioning material from within a background of uncorrelated natural radiation. A detector capable of distinguishing neutrons from gammas, as well as offering a fast response time typically has better capability for detecting the distinctive time-correlated events indicative of the presence of fissioning nuclei.

The ability to detect gamma rays and/or neutrons is a vital tool for many areas of research. Gamma-ray/neutron detectors allow scientists to study celestial phenomena and diagnose medical diseases, and they have been used to determine the yield in an underground nuclear test. Today, these detectors are important tools for homeland security, helping the nation confront new security challenges. The nuclear non-proliferation mission requires detectors capable of identifying diversion of or smuggling of nuclear materials. Government agencies need detectors for scenarios in which a terrorist might use radioactive materials to fashion a destructive device targeted against civilians, structures, or national events. To better detect and prevent nuclear incidents, the Department of Energy (DOE) and the Department of Homeland Security (DHS) are funding projects to develop a suite of detection systems that can search for radioactive sources in different environments.

One particularly useful type of radiation detection, pulse shape discrimination (PSD), which is exhibited by some organic scintillators, involves subtle physical phenomena which give rise to the delayed luminescence characteristic of neutrons, providing a means of distinguishing neutrons from the preponderance of prompt luminescence arising from background gamma interactions. The mechanism by which this occurs begins with the excitation process which produces excited singlet (S1) and excited triplet (T1) states nonradiatively relaxes to the configuration, as shown in FIG. 1. In FIG. 1, the basic physical processes leading to the delayed fluorescence characteristic of neutron excitation of organics with phenyl groups is shown.

Since the triplet is known to be mobile in some compounds, the energy migrates until the collision of two triplets collide and experience an Auger upconversion process, shown as Equation 1: T ₁ +T ₁ →S ₀ +S ₁  Equation 1

In Equation 1, T1 is a triplet, S₀ is the ground state, and S₁ is a first excited state. Finally, the delayed singlet emission occurs with a decay rate characteristic of the migration rate and concentration of the triplet population, which is represented as Equation 2: S ₁ →S ₀ +hv  Equation 2

In Equation 2, hv is fluorescence, while S₀ is the ground state and S₁ is a first excited state. The enhanced level of delayed emission for neutrons arises from the short range of the energetic protons produced from neutron collisions (thereby yielding a high concentration of triplets), compared to the longer range of the electrons from the gamma interactions. The resulting higher concentration of triplets from neutrons, compared to gamma interactions, leads to the functionality of PSD. The observation of PSD is believed to be in part related to the benzene ring structure, allowing for the migration of triplet energy.

FIG. 2A shows a typical plot of logarithmic population versus linear time (ns) for stilbene. Population is the singlet excited state population, which is proportional to the output of light from a test crystal under examination, in this case a stilbene crystal, after the crystal it is excited by high energy radiation. As can be seen from the plot, some light is produced by the crystal almost immediately, referred to as prompt luminescence, and other light is produced from the crystal over a period of time, referred to as delayed luminescence. Generally, the plot for each type of radiation will have a steep component 202 and a tail component 204, where the differentiation point 206 between the two is defined in the region where the slope of the line changes dramatically. In this example, the steep component 202, tail component 204, and differentiation point 206 for the Neutron curve is labeled. Note that the steep component, tail component, and differentiation point for the Gamma curve is different for stilbene, and other compounds which possess good PSD properties. Compounds which do not possess good PSD properties will generally not have substantial differences in the curves plotted for Gamma and Neutron radiation. The upper line in the plot shown in FIG. 2A is a Neutron-induced scintillation pulse shape, while the lower line is a Gamma-induced scintillation pulse shape. As can be seen, stilbene is able to differentiate between the Neutron and Gamma pulse shapes, and produces noticeably different luminescence decay lineshapes for each radiation type. However, not every compound has this ability to separate between Gamma and Neutron pulse shapes, and therefore compounds which do are very useful for PSD, as Gamma and Neutron luminescence decay plots have different pulse shapes for these compounds.

Once the population versus time plot has been determined for each test crystal under examination, if it appears that there is PSD for the crystal type, the area (Q_(S)) under the tail component of the curve for each type of radiation is calculated, along with the area (Q_(F)) under the entire line for each type of radiation. By dividing the total area (Q_(F)) into the tail area (Q_(S)), a scatterplot of the ratio of charge versus the pulse height can be produced, as shown in FIG. 2B for stilbene. FIG. 2B shows a plot of the ratio of charge (Q_(S)/Q_(F)) versus the pulse height, which correlates to an output of a light detector, such as a photomultiplier tube. The x-axis represents the pulse height, which is proportional to the energy of the event. Gamma events correspond to light produced by Compton electrons generated in the detector material. Neutron events correspond to proton recoils in the detector material; lower energy proton recoil events correspond to “glancing angle” interactions between the neutron and proton in the detector material, while a high energy “knock-on” interaction between a neutron and a proton will produce a higher energy event.

Referring to FIG. 2B, at hv equal to about 1600V, conventional scintillators utilizing stilbene exhibit a neutron-to-gamma (n°/γ) separation S of about 0.132. The greater the separation S of neutron-to-gamma, the better PSD performance can be expected.

It is with these scatter plots that good PSD separation can be determined, which is defined as PSD separation, S, which is the gap between the mean ratio of charge (Q_(S)/Q_(F)) for Gamma and the mean ratio of charge (Q_(S)/Q_(F)) for Neutron taken over an extended period of time. The higher this separation, S, is, the better the compound is at PSD separation.

It is generally accepted in the prior art that stilbene offers good PSD. However, stilbene, generally grown from melt, is difficult to obtain. Therefore, a number of other organic molecules have been examined. Unfortunately, most research in this area has concluded that many known liquid and solid materials, including many compounds having benzene rings, do not possess PSD properties comparable to single-crystal stilbene. Despite the difficulty in identifying compounds with suitable PSD properties, the inventors previously succeeded in demonstrating several exemplary compounds with suitable PSD properties and capable of being grown from solution, including 1-1-4-4-tetraphenyl-1-3-butadiene; 2-fluorobiphenyl-4-carboxylic acid; 4-biphenylcarboxylic acid; 9-10-diphenylanthracene; 9-phenylanthracene; 1-3-5-triphenylbenzene; m-terphenyl; bis-MSB; p-terphenyl; diphenylacetylene; 2-5-diphenyoxazole; 4-benzylbiphenyl; biphenyl; 4-methoxybiphenyl; n-phenylanthranilic acid; and 1-4-diphenyl-1-3-butadiene.

Moreover, crystals such as stilbene, generally grown from melt, are difficult to obtain. Therefore, organic liquid scintillator cocktails comprised of an aromatic solvent, such as toluene, a primary and a secondary fluor, have been developed and are commercially available, however, liquid scintillators do not exhibit PSD properties comparable t single-crystal stilbene, and are also hazardous to field, because these compounds typically include flammable, toxic, and otherwise hazardous materials that limit application to sensitive environments such as aviation, military applications, medical applications, and etc. Moreover, the above crystals, especially when grown from solution, tend to be relatively fragile, making safe and efficient transport difficult.

Accordingly, it would be beneficial to provide organic materials comparable to or better than stilbene in relation to PSD properties for neutron radiation detection, but in a form that is easier to fabricate into large monoliths which are durable, and which do not introduce hazardous material into the radiation detection process.

SUMMARY

In one general embodiment, a scintillator material includes a polymer matrix; and a primary dye in the polymer matrix, the primary dye being a fluorescent dye, the primary dye being present in an amount of 5 wt % or more; wherein the scintillator material exhibits an optical response signature for neutrons that is different than an optical response signature for gamma rays.

In another general embodiment, a scintillator material includes a polymer matrix; and a primary dye in the polymer matrix, the primary dye being a fluorescent dye, the primary dye being present in an amount greater than 10 wt %.

Other aspects and embodiments of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a mechanism for delayed photoluminescence according to the prior art.

FIG. 2A shows a plot of Population versus Time for stilbene according to the prior art.

FIG. 2B shows a plot illustrating PSD separation of stilbene according to the prior rt.

FIG. 3 shows a simplified schematic layout of a system that may use crystals described herein, according to one embodiment.

FIG. 4A shows a comparative graphical representation of gamma/neutron separation plotted according to charge ratio versus pulse height for an organic plastic scintillator and stilbene, according to one embodiment.

FIG. 4B shows a comparative graphical representation of integrated gamma/neutron separation plotted for an organic plastic scintillator with FOM 3 and an Eljen liquid with FOM 2.21, according to one embodiment.

FIG. 5 shows a comparative graphical representation of integrated gamma/neutron separation plots for organic plastic scintillators utilizing different phases and/or concentrations of 2,5-diphenyl oxazole (PPO) and diphenyl anthracene (DPA), according to various embodiments.

FIG. 6 shows a plot comparing integrated gamma/neutron separation for scintillator systems including a plastic scintillator and a primary fluor, a plastic scintillator, a primary fluor and a secondary fluor, an Eljen liquid, and a stilbene crystal, according to various embodiments.

FIG. 7A shows a plot of signal intensity versus pulse integral for an organic plastic scintillator employing 30% 2,5-diphenyl oxazole (PPO) as a primary fluor versus an organic plastic scintillator employing 30% 2,5-diphenl oxazole (PPO) as a primary fluor and 0.2% diphenyl anthracene (DPA) as a secondary fluor.

FIG. 7B shows a plot of photoluminescence intensity versus wavelength for an organic plastic scintillator employing 30% 2,5-diphenyl oxazole (PPO) as a primary fluor versus a scintillator employing 30% 2,5-diphenyl oxazole (PPO) as a primary fluor and 0.2% diphenyl anthracene (DPA) as a secondary fluor.

FIG. 8 shows a plot of PSD characteristics versus fluor concentration for PPO and DPA solubilized in xylene solution, according to one embodiment.

FIG. 9 depicts experimental PSD patterns showing increase of neutron-gamma peak separation at increasing PPO concentration in a polyvinyl toluene (PVT) polymer matrix, according to one embodiment.

FIG. 10 is a plot depicting general dependence of neutron-gamma separation measured in the whole range of concentrations up to the limit of the PPO solubility.

FIG. 11 is a flowchart of a method, according to one embodiment.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified. The term “about” as used herein refers to ±10% of the denoted value, unless otherwise noted herein.

The description herein is presented to enable any person skilled in the art to make and use the invention and is provided in the context of particular applications of the invention and their requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art upon reading the present disclosure, including combining features from various embodiment to create additional and/or alternative embodiments thereof.

Moreover, the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

Unless otherwise noted herein, all percentage values are to be understood as percentage by weight (wt %) Moreover, all percentages by weight are to be understood as disclosed in an amount relative to the bulk weight of an organic plastic scintillator material, in various approaches.

The following description describes several embodiments relating to the use of the fabrication of polymer scintillator materials with distinctively different scintillation pulse shapes resulting from neutron and gamma excitation, respectively.

Traditionally, gamma excitation in the conventional organic plastic scintillator materials using conventional fluor concentrations in the range of about 0.1-5 wt % fluor have exhibited low excitation density, and weak delayed luminescence. Thus, the ability to distinguish high-energy neutron radiation from gamma radiation has previously been achievable only with liquid scintillators, which are difficult to field, and organic single crystals, which can be fragile and difficult to produce in very large volumes.

Accordingly, the inventive approaches detailed in the present disclosures have not been demonstrated in the past, presumably since very few fluors are soluble at >5 wt % in a polymer matrix. Indeed, the traditional polymer scintillator materials have been limited to including fluors in concentrations which demonstrably lack PSD characteristics, discouragingly suggesting that organic plastic polymer materials would be unsuitable for use as a scintillator material capable of exhibiting PSD characteristics.

However, the inventors of the presently disclosed scintillator have surprisingly discovered that in some embodiments, loading an organic plastic polymer material with high concentrations of fluors, (≥10 wt %) results in sufficient migration of triplet excitation within the or al that the higher excitation density produced by neutron-induced proton recoil results in delayed singlet luminescence from triplet-triplet annihilation. Without wishing to be bound to any particular theory, it is speculated that loading an organic plastic scintillator material with ≥10 wt % of a primary fluor and ≤1 wt % of a secondary fluor with better spectral match to standard photomultipliers may produce an even more effective pulse shape discrimination material. Thus, the use of plastic scintillator material for this application enables passive detection of high-energy neutron radiation as distinguishable from gamma radiation, as well as active interrogation methods, according to various approaches.

In particular, recent studies conducted with organic crystals showed that the main reason for the absence of PSD in mixed systems results from the excitation traps formed by a lower-band-gap fluorescent impurity (fluor) present in the host material (solvent) at low concentration. Moreover, increasing the concentration of the fluor can provide conditions suitable for formation of a network for excitation energy migration and triplet annihilation, and may lead to the appearance of PSD comparable to that typical for pure single crystals of the fluor, in various approaches.

The present disclosure introduces the results of studies conducted with mixed liquid and plastic scintillating systems. Analysis of the results shows that explanations of the conditions leading to the formation of PSD in crystals and liquids can be similarly applied to the mixed plastic systems. The properties of the exemplary plastics scintillators fabricated with efficient neutron/gamma discrimination are discussed in comparison with commercially available liquid and single crystal organic scintillators.

In one general embodiment, a scintillator material includes a polymer matrix; and a primary dye in the polymer, the primary dye being a fluorescent dye, the primary dye being present in an amount of 5 wt % or more; wherein the scintillator material exhibits an optional response signature for neutrons that is different than an optical response signature for gamma rays.

In another general embodiment, a scintillator material includes a polymer and a primary dye in the polymer matrix, the primary dye being a fluorescent dye, the primary dye being present in an amount greater than 10 wt %.

General Scintillator-Based Radiation Detector System

FIG. 3 depicts a simplified spectroscopy system according to one embodiment. The system 300 comprises a scintillator material 302, such as of a type described herein, and which is referred to herein interchangeably as a scintillator. The system 300 also includes a photodetector 304, such as a photomultiplier tube or other device known in the art, which can detect light emitted from the scintillator 302, and detect the response of the material to at least one of neutron and gamma ray irradiation.

The scintillator 302 produces light pulses upon occurrence of an event, such as a neutron, a gamma ray, or other radiation engaging the scintillator 302. As the gamma ray, for example, traverses the scintillator 302, photons are released, appearing as light pulses emitted from the scintillator 302. The light pulses are detected by the photodetector 304 and transduced into electrical signals that correspond to the pulses. The type of radiation can then be determined by analyzing the integral of the light pulses and thereby identifying the gamma ray energy absorbed by the scintillator.

In some embodiments, the system 300 may be, further comprise, or be coupleable/coupled to, a processing device 306 for processing pulse aces output by the photodetector 304. In other embodiments, the system 300 may include a processing device that receives data from a photodetector that is not permanently coupled to the processing device. Illustrative processing devices include microprocessors, field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), computers, etc.

The result of the processing may be output and/or stored. For example, the result may be displayed on a display device 308 in any form, such as in a histogram or derivative thereof.

The program environment in which one embodiment of the invention may be executed illustratively incorporates one or more general-purpose computers or special-purpose devices such hand-held computers. Details of such devices (e.g., processor, memory, data storage, input and output devices are well known and are omitted for the sake of clarity.

It should also be understood that the techniques of the present invention might be implemented using a variety of technologies. For example, the methods described herein may be implemented in software running on a computer system, or implemented in hardware utilizing one or more processors and logic (hardware and/or software) for performing operations of the method, application specific integrated circuits, programmable logic devices such as Field Programmable Gate Arrays (FPGAs), and/or various combinations thereof. In particular, methods described herein may be implemented by a series of computer-executable instructions residing on a storage medium such as a physical (e.g., non-transitory) computer-readable medium. In addition, although specific embodiments of the invention may employ object-oriented software programming concepts, the invention is not so limited and is easily adapted to employ other forms of directing the operation of a computer.

Portions of the invention can also be provided in the form of a computer program product comprising a physical computer readable medium having computer code thereon. A computer readable medium can include any physical medium capable of storing computer code thereon for use by a computer, including optical disks such as read only and writeable CD and DVD, magnetic memory or medium (e.g., hard disk drive), semiconductor memory (e.g., FLASH memory and other portable memory cards, etc.), etc.

Polymer

The inventive organic plastic scintillator system as encompassed by the present disclosures may include any suitable polymer as the plastic component. Particularly suitable re rigid, durable, transparent plastics, possessing an aromatic structure consisting of pi-conjugated rings, and capable of supporting high concentrations of primary, secondary, tertiary, and etc. fluors, e.g. with a total concentration in the range of about 5-75 wt % fluor, according to some embodiments. In a preferred embodiment, the organic plastic component may include a polymer comprising polyvinyltoluene (PVT). In other embodiments, similar polymers may be utilized, such as polystyrene (PS), polyvinyl xylene (PVX), polymethyl, 2,4-dimethyl, 2,4,5-trimethyl styrenes, polyvinyl diphenyl, polyvinyl naphthalene, polyvinyl tetrahydronaphthalene polymers, other complex aromatic polymers, and certain non-aromatic polymers capable of solubilizing high fluor concentrations, etc. as would be understood by one having ordinary skill in the art upon reading the present descriptions.

As described herein, suitable polymers may be preferably at least 95% light transmissive in a wavelength of interest, e.g., a wavelength emitted by one or more fluors present in the organic plastic scintillator system, in some embodiments.

Moreover, the polymer may be provided as a liquid polymer matrix, a non-liquid polymer matrix, a dry powder, etc. as would be understood by one having ordinary skill in the art upon reading the present descriptions. Moreover, in various approaches the polymer matrix may include aromatic functional groups, such as phenyl groups, among others.

Fluors

Primary fluors suitable for use in the presently disclosed scintillator system include any fluor as known in the art and capable of exhibiting characteristics of pulse-shape discrimination as described herein. Moreover, the primary fluor of the exemplary organic plastic scintillator system is present in high concentration, e.g. about 5 wt % or more, in one embodiment. In preferred embodiments, the primary dye may be present in an amount of 20 wt % or more, and in particularly preferred embodiments, the primary dye may be present in an amount ranging from about 20 wt % to about 75 wt % or an amount ranging from about 30 wt % to about 75 wt %. As discussed herein, the concentrations of fluor are described relative to a weight of the bulk scintillator material, in various embodiments.

In one particular embodiment, a scintillator system may include a polymer matrix and a primary fluor disposed in the polymer matrix. Moreover, the primary fluor may be a fluorescent dye present in an amount of 5 wt % or more, and such fluorescent dye results in the scintillator material exhibiting an optical response signature for neutrons that is different than an optical response signature for gamma rays.

Accordingly, where primary fluors are present in high concentration in the exemplary organic plastic scintillator system, a corollary principle is that the solubility of the fluor in the polymer is preferably high. In one embodiment, for example, the polymer may be characterized by having a solubility of about 5 wt % or more with respect to particular fluor.

Moreover, in some approaches the primary fluor may include multiple dyes. In further approaches the primary fluor may include multiple fluorescent dyes.

Moreover still, primary fluor may be incorporated into the polymer according to any suitable mechanism. For example, in one embodiment the primary fluor may be suspended in the polymer matrix and dispersed to an approximately uniform distribution. In other embodiments, the primary fluor may be crosslinked to the polymer matrix. In still other embodiments, the primary fluor may be copolymerized with the polymer matrix, and/or with another component of the polymer matrix, etc. as would be understood by one having ordinary skill in the art upon reading the present descriptions. Of course, other arrangements of fluor and polymer matrix may be utilized without departing from the scope of the present descriptions.

The secondary fluor of the exemplary plastic scintillator system is characterized by wavelength-shifting qualities, such that in the presence of another fluor, particularly a primary fluor present in high concentration in a plastic scintillator system, PSD characteristics of the plastic scintillator system with the primary fluor and the secondary fluor in combination are superior to PSD characteristics of a plastic scintillator system having the same primary fluor exclusively present in high concentration, according to one embodiment.

Suitable secondary fluors include any fluor characterized by wavelength-shifting as described herein, and several exemplary embodiments may utilize secondary fluors such as diphenyl anthracene (DPA), tetraphenyl butadiene (TPB) 1,1,4,4-tetraphenyl-1,3-butadiene, 1,4-Bis(5-phenyl-2-oxazolyl)benzene (POPOP), p-bis(o-methylstyryl)benzene, 1,4-bis-2-(4-methyl-5-phenyloxazolyl)benzene, 2,2′-p-phenylenebis(5-phenoxazole), diphenylstilbene, 1,3,5-triaryl-2-pyrazolines, 4-(n-butylamino)-2-(4-methoxyphenyl)benzo[b]pyrylium perchlorate, sodium salicylate, 1,4-bis(2-methylstyryl)benzene (Bis-MSB), 7-dimethylamino-4-methyl-2-quinoline, 7-amino-4-methylcoumarin, 4,6-dimethyl-7-ethylamino coumarin, etc. as would be understood by one having ordinary skill in the art upon reading the present descriptions. Particularly preferred secondary fluors include DPA, TPB, POPOP, and Bis-MSB according to one embodiment.

Regarding the concentration of the secondary fluor, as described herein the exemplary organic plastic scintillator system may include secondary fluor in a low concentration in order to maximize the beneficial wavelength-shifting effects on PSD performance. For example, in one embodiment the secondary fluor may be present in an amount of about 2 wt % or less. As described herein, secondary fluors may be present in an amount described relative to a weight of the bulk scintillator material.

Particularly impressive PSD values have surprisingly been obtained with a plastic scintillator embodiment including polyvinyl toluene (PVT) with 30% 2,5-diphenyl oxazole (PPO) as primary fluor and 0.5% diphenyl anthracene (DPA), as secondary fluor, according to one embodiment. Particularly surprising is the solubility of PPO in the PVT polymer, which allows for excellent PSD characteristics described herein.

One preferred embodiment of a PSD plastic can by formed by combining: 0.1-1% Benzoyl peroxide (initiator), 30% 2,5-diphenyl oxazole (PPO) as primary fluor, 0.2-0.5% diphenyl anthracene (DPA), or tetraphenyl butadiene (TPB) as secondary fluor and balance vinyl toluene. The process of creating the plastic corresponding to this embodiment may include: adding the materials listed above to a container in a glovebox under an N₂ atmosphere, such as a wide mouth glass jar in one exemplary embodiment, placing the container in an oven at 80 C, and allowing the jar to sit in the oven undisturbed for four days, after which it is cooled to room temperature in ambient conditions. The resultant polymer is rigid, substantially transparent and offers excellent scintillation properties for pulse shape discrimination.

Scintillator Fabrication

Various embodiments may employ any known scintillator material without departing from the scope of the present disclosure. However, several preferred approaches for fabricating scintillators with suitable PSD characteristics from organic plastic are described below.

In one embodiment, liquid scintillator mixtures were fabricated from anhydrous p-xylene (>99%), 2,5-diphenyloxazole (PPO, 99%), and 9,10-diphenylanthracene (DPA, >98%) in the oxygen-free atmosphere of a nitrogen-filled glovebox. PPO was used as received, DPA was stirred for 0.5 h in warm acetone, collected by filtration, dried, and stored in nitrogen atmosphere prior to sample preparations. The liquid mixtures of required concentrations were transferred into sealed 50 mm×10 mm cylindrical quartz cuvettes and subsequently used for further measurements.

In another approach, plastic scintillator mixtures were fabricated as follows. Vinyl toluene was filtered through a chromatographic support material to remove inhibitor prior to polymerization. Filtered vinyl toluene and an initiator, e.g. benzoyl peroxide, were sparged with nitrogen for 40 minutes and stored in sealed containers in a glovebox refrigerator at −20 C. To conduct polymerization, required amounts of PPO and DPA were weighed in a glovebox into 20 mL scintillation vials, initiator (10-30 mg) and vinyl toluene were then added to make up the final weight proportions of polymer parts. The vials were tightly sealed, removed from the glovebox, and placed in an oven at 80 C. Two hours later they were shaken to ensure complete mixing, and then held at 80 C for a total of 96 hours. After cooling to room temperature the glass was scored and broken with a mallet to remove the bare scintillator part.

Of course, the above fabrication methodologies are provided only by way of example, and organic plastic scintillator systems comprising polymers other than PVT and/or fluors other than DPA/PPO may be fabricated under similar conditions, but taking account for slight variations in various approaches, e.g. to temperature, incubation time, amount of respective components, etc. as would be understood by a skilled artisan reading the present descriptions.

Furthermore, the present descriptions also encompass methods for fabricating scintillator materials as described herein, as particularly represented by FIG. 11, in various approaches. FIG. 11 depicts a method 1100. As will be appreciated by the skilled artisan reading the present descriptions, the method 1100 may be performed in any environment, including those depicted in FIGS. 1-10, among others. Regardless of environment, the method 1100 is characterized by operation 1102, where a scintillator precursor mixture is placed in a heating vessel, and subsequently heated until a polymerization process has completed in operation 1104. In one embodiment, the scintillator precursor mixture is a combination of about 60-95 wt % of a monomer, about 5-40 wt % of a primary fluor, and about 0.1 wt % of a free radical initiator. Any monomer capable of polymerizing and solvating the primary fluor in an amount ranging from about 5-40 wt % may be employed in the exemplary fabrication process, according to one embodiment and as would be understood by one having ordinary skill in the art upon reading the present descriptions.

In one particular embodiment, the fabrication process may include combining about seven grams of vinyl toluene, about 2.94 grams of PPO, and about 0.01 grams of benzoyl peroxide in a heating vessel, mixing the combination, and heating the mixture at about 80° C. for approximately 96 hours.

In another embodiment, the fabrication process may include combining about seven grams of vinyl toluene, about 2.94 grams of PPO, about 0.05 grams of DPA, and about 0.01 grams of benzoyl peroxide in a heating vessel, mixing the combination, and heating the mixture at about 80° C., for approximately 96 hours.

Notably, the solid structure may include any of the structures described herein, and the primary fluor may include any fluor as described herein, according to some approaches.

Experimental Results: Photoluminescence

Growth and characterization of mixed single crystals led to an understanding of the mechanisms of excited state migration and interaction, prompting an explanation of compositions of polymer scintillator with sufficient fluor to reach the percolation threshold whereupon triplet excitation is able to migrate and annihilate. Experiments with complex liquid mixtures lead to findings that high loading with fluors helps improve pulse shape discrimination (PSD) in such organic scintillators as well. So far, the polymer scintillators offering PSD exhibit a figure of merit (FOM) for PSD of 3, compared to >4 for certain single crystal organics, such as stilbene, etc. This performance metric is already sufficient to distinguish neutrons from gammas down to the few hundred keV/gamma equivalent regime, and will be very useful for non-proliferation, homeland security and safeguards applications.

Photoluminescence (PL) spectra were measured under UV excitation using a commercial Spex Fluoromax-2 spectrometer. The scintillation light yield efficiency was evaluated from the position of the Compton edge in the ¹³⁷Cs spectra, in which 480 keVee (electron-equivalent energy) was defined by 50% of the Compton edge peak. Neutron detection properties of samples were studied using a ²⁵²Cf source shielded with 5.1 cm of lead, which reduced the gamma rates to the same order of magnitude as neutrons, to irradiate liquid or plastic samples coupled to Hamamatsu R6231-100-SEL photomultiplier tube (PMT). The signals collected at the MIT anode were recorded using a 14-bit high-resolution waveform CompuScope 14200 digitizer with a sampling rate of 200 MS/s, for offline analysis.

The ability of scintillators to discriminate between the neutrons and gamma rays emitted from the ²⁵²Cf source was evaluated using the charge integration method. In particular, waveforms were numerically integrated over two time intervals: Δt_(Total) and a subinterval Δt_(Tail), corresponding to the total charge (Q_(Total)) and the delayed component (Q_(Tail)) of the signal, respectively. The value of the ratio of charge R=Q_(Tail)/Q_(Total) for the two time intervals indicated whether the considered event was likely produced by a neutron (high R value) or a gamma ray (small R value). Quantitative evaluation of PSD was made using Figures of Merit (FOM) as represented in Equation 3, where S is the separation between gamma and neutron peaks, and δ_(gamma) and δ_(neutron) are the full width at half maximum (FWHM) of the corresponding peaks, as shown in FIG. 5, according to one embodiment. FOM=S/(δ_(gamma)+δ_(neutron))  Equation 3

The experimental separation S was calculated as a difference between the mean delayed light fraction, (Q_(Tail)/Q_(Total)) for neutrons and gammas taken as a normal distribution in PSD over a specified energy range. In total 40000 events were collected for each scintillator sample, with approximately 20% of the statistics used for FOM calculation in the energy range near the Compton edge.

A reasonable definition for well separated Gaussian distributions of similar population sizes is shown in Equation 4, below, where σ is the standard deviation for each corresponding peak. σ>3(σ_(gamma)+σ_(neutron))  Equation 4

Noting that where FWHM≈2.36, a reference parameter FOM≥3(σ_(gamma)+σ_(neutron))/2.36(σ_(gamma)+σ_(neutron)) was used to define efficient PSD in the tested samples. The experimentally determined efficiency value was ≈1.27

One characteristic of the exemplary scintillator material described herein is that the system exhibits an optical response signature for neutrons that is different than an optical response signature for gamma rays, according to one embodiment. In particular, the neutron optical response signature may be in the range of about 600-800 keV gamma equivalent, according to one embodiment.

FIGS. 4A-4B, show examples of PSD patterns measured in some organics with phenyl groups according to several embodiments. FIG. 4A shows a comparison of the charge ratio Q_(O)/Q_(F) as exhibited by one embodiment of an organic plastic scintillator as described in the present disclosures, and a scintillator utilizing a stilbene crystal. As can be seen from the figure, the inventors have successfully created organic plastic scintillator embodiments that exhibit similar PSD characteristics as single crystal systems employing stilbene as the scintillating material. Accordingly, it is possible to generate a more rigid and durable, scintillator material from organic plastic while retaining the advantageous neutron and gamma radiation separation characteristics of more expensive and fragile alternatives such as liquid and single crystal scintillators.

FIG. 4B depicts comparative FOM readings taken from an exemplary organic plastic scintillator exhibiting PSD as described herein and according to one embodiment as compared to a FOM reading of a LLNL Eljen liquid scintillator, according to one embodiment. When comparing FOM readings from FIG. 4B, both of which used test samples of approximately the same volume, a clear improvement in the FOM levels is apparent. Particularly, the FIG. 4B illustrates a FOM reading of 3 taken from a LLNL plastic scintillator, according to one embodiment. This FOM reading shows PSD approaching the level of typical liquid and single crystal organic scintillators, thus improving neutron/gamma discrimination properties of both liquid and plastic scintillators in the embodiments shown, among others within the scope of the present descriptions.

The discovery that organic plastic scintillators are suitable alternatives for more dangerous, expensive, burdensome, etc. systems such as single crystal scintillators and liquid scintillators led the inventors to experiment with a variety of polymer and fluor candidates. With reference to FIG. 5, several exemplary embodiments of organic plastic scintillators as described herein are depicted according to FOM performance of various embodiments including different fluor(s) in a range of concentrations, in some approaches.

As shown in FIG. 5, the top row of graphs represent measurements that were taken from liquid scintillator systems comprising diphenyl anthracene (DPA) as a fluor. Conversely, the bottom row of graphs represent measurements that were taken from scintillator systems comprising 2,5-diphenyl oxazole (PPO) as a fluor. Each fluor exhibits characteristic FOM as a crystal, shown in FIG. 5 as FOM=4.42 for DPA crystal and FOM=1.76 for PPO crystal powder, respectively.

Moreover, as can be seen from FIG. 5, fluor FOM generally decreases as a function of concentration in solution. For example, a solution of about 2.2% DPA yields a FOM value of about 3.79, according to one embodiment, while a solution of about 0.05% DPA exhibits a FOM value of about 1.12, according to another embodiment, and a solution of about 0.01% DPA exhibits a FOM value of about 0.58, according to yet another embodiment.

Similarly, liquid solutions of PPO yield FOM values that decrease as a function of PPO concentration. For example, as shown in FIG. 5 a solution of about 37.5% PPO yields a FOM value of approximately 3.26, while a solution of about 0.05% PPO yields a FOM value of about 1.52, and a solution of about 0.01% PPO yields a FOM value of about 0.71, according to several embodiments.

Referring now to FIG. 6, a plot comparing integrated gamma/neutron separation for scintillator systems including a plastic scintillator and a primary fluor, a plastic scintillator, a primary fluor and a secondary fluor, an Eljen liquid, and a stilbene crystal is shown, according to various embodiments.

Panel A of FIG. 6 depicts PSD characteristics of an organic plastic scintillator system including PPO as a primary fluor, and no secondary fluor. As can be seen from the figure, this system exhibits a FOM value of about 2.82, according to one embodiment. Comparatively, panel B shows a similar system comprising an organic plastic scintillator with a primary fluor including PPO and further including a secondary fluor. As can be seen from FIG. 6, this system exhibits superior FOM values compared to the similar system lacking a secondary fluor, and in one embodiment exhibits a FOM value of approximately 3.31.

Further still, Panels C and D of FIG. 6 provide reference points from which to compare the performance of the exemplary organic plastic scintillator systems as described herein against the performance of liquid and single crystal stilbene scintillator systems, respectively. As can be seen from the figure, the liquid scintillator system shown in Panel C exhibits a FOM value of approximately 3.21, while the single crystal stilbene scintillator system shown in Panel D exhibits a FOM value of about 4.70, according to some embodiments. Notably, the exemplary organic plastic scintillator system including PPO as a primary fluor and a secondary fluor outperforms the liquid scintillator system in terms of FOM value, and approximates the performance of the industry-standard single crystal stilbene system. Accordingly, high quality PSD characteristics may be imparted to modern scintillator systems without incorporating expensive, fragile, and/or hazardous materials of the conventional scintillator system.

Referring now to FIGS. 7A and 7B, the comparative effects of including multiple fluors in a scintillator crystal are shown, according to one embodiment.

As may be seen particularly from FIG. 7A, a plastic scintillator having, e.g. 30% w/v PPO as a primary fluor is capable of neutron detection. Moreover, neutron detection may be additionally improved by incorporating a low concentration of a secondary fluor, where the secondary fluor is characterized by emitting photons of lower energy (longer wavelength) than photons emitted by the primary fluor, according to one embodiment.

As shown in FIG. 7B, embodiments of plastic scintillators containing only the primary fluor PPO exhibit an emission spectrum with a major peak near 360 nm. By contrast, embodiments of plastic scintillators containing primary and secondary fluor, such as DPA, exhibit emission spectra characterized by a peak shift to a longer wavelength (lower energy). Moreover, including a secondary fluor also results in greater luminescence than in systems including only a primary fluor, according to one embodiment.

Moreover, including high concentrations of primary fluor improves PSD behavior, but at higher concentrations disadvantageous self-absorption of emitted photons traps the light and prevents its emission from the scintillator. In one approach, including a secondary fluor with a lower emission wavelength than the primary fluor shifts the energy of the emitted photon and preventing unfavorable absorption thereof by the primary fluor molecules. Preferably, secondary fluor may be included in such low concentration that self-absorption is negligible, thereby circumventing the disadvantages inherent to high concentrations of a single fluor, according to one embodiment.

Referring now to FIG. 8, the dependence of PSD on fluor concentration is shown as measured in one embodiment including PPO as a primary fluor and DPA as a secondary fluor through the entire range of respective solubility in xylene.

For both types of solutions, there is a region of very low fluor concentrations (<1 μmole/g solution, or ˜0.02 wt. %) with negligibly small PSD. Increasing the concentration leads to a gradual enhancement of PSD which, despite the large difference in the solubility, surprisingly exhibits a similar slope for both fluors up to a molecular concentration of about 10 μmole/g solution. It is further interesting to note that the separation of the PSD curves unexpectedly occurs at a concentration corresponding to the maximum light yield for both PPO and DPA (˜10 μmole/g), as particularly shown in panel B of FIG. 8, according to one embodiment.

This decrease of the scintillation light efficiency at increasing fluor concentration (concentration quenching) is ascribed to the formation of excimers (S₀S₁). Therefore, the separation of the PSD curves may relate to different kinetics of these processes for different types of the molecules. For example, in one embodiment a rise in PSD to efficient values above a concentration threshold as measured in DPA and PPO solutions is similar to that observed in mixed crystal scintillation systems, which exhibit PSD behavior according to the simple model of energy transfer and triplet-triplet annihilation shown in FIG. 1.

Accordingly, at very small concentrations, excited singlet states of the solute molecules still produce scintillation light, while excited triplets behave more like energy traps, since direct fluorescence from the triplets is effectively forbidden.

Meanwhile, at relatively large intermolecular distances and low probability of collisions, e.g. in dilute solutions, fluor molecules cannot interact, thus leading to quenched triplet migration, recombination, and a resulting degradation, or even worse, absence of PSD behavior. At the higher concentrations, increased probability of triplet-triplet collisions leads to the enhancement of the delayed light and a rise of the PSD above a certain concentration threshold corresponding to the establishment of a continuous network of interacting fluor molecules.

Several exemplary organic plastic scintillators with varied fluor concentration were analyzed, and some results of several exemplary embodiments are shown in FIG. 9. As evidenced by FIG. 9, a general trend is observable wherein quality of separation between neutron radiation signals and gamma radiation signals exhibits improved resolution with increasing primary fluor concentration. For example, as shown in FIG. 9 one embodiment of an organic plastic scintillator system including only 1 wt % PPO as a primary fluor exhibits no resolution of neutron and gamma radiation signals, respectively, and therefore is incapable of discriminating between the two as required for suitable PSD performance. At about 6 wt % PPO, neutron radiation and gamma radiation signals begin to resolve, but still insufficiently to achieve desirable PSD characteristics, according to some embodiments.

In some approaches, true separation between neutron radiation and gamma radiation signals can be seen at primary fluor concentrations of about 10 wt %, and improve with increased primary fluor concentration to about 15 wt %, as shown in FIG. 9, according to one embodiment. Moreover, separation continues to improve with increasing primary fluor concentration, and signals are completely distinguishable according to one embodiment including about 30 wt % primary fluor. Of course, further increases in primary fluor concentration may be expected to further increase signal separation and hence PSD performance, but the inventors have observed that primary fluor concentrations above about 75 wt % may exhibit inhibitory effects on signal resolution and PSD performance. Accordingly, primary fluor concentrations in a range of about 10 wt % to about 75 wt % relative to the bulk weight of the scintillator material are preferred, in some approaches.

FIG. 10 further depicts the generally observed relationship between fluor concentration and PSD performance with particular reference to fraction of delayed light for both neutron and gamma radiation, according to one embodiment. As can be seen from FIG. 10, organic plastic scintillators as described herein and utilizing PPO as a primary fluor exhibit a direct relationship between PPO concentration and amount of delayed light, whether from gamma or neutron radiation. Accordingly, FOM values and corresponding PSD characteristics also improve with increasing primary fluor concentration, in some embodiments.

Accordingly, the fixed position of molecules in a polymer matrix indicate advantageous performance where the concentration of a fluor required for efficient PSD is closer to that in mixed crystals rather than in liquid solutions, in various approaches.

Digital Processing for Pulse Shape Discrimination

In one digital processing approach, signals corresponding to subset of the events are selected and processed.

Yet another approach includes processing two or more integration windows (e.g., 0-τ₁, τ₁-τ₂), and employing this ratio to deduce a pulse shape discrimination factor, derived from each individual scintillation pulse.

In any approach, and particularly approaches utilizing digitization as described herein, the exemplary scintillator system employing an organic plastic polymer may further include additional components. In one embodiment, for example, the exemplary scintillator system may include a processor, e.g. for performing a discrimination method for processing an output of the photodetector using pulse shape discrimination for differentiating responses of the material to the neutron and gamma ray irradiation. In another embodiment, the exemplary scintillator system may additionally and/or alternatively include a photodetector, e.g. for detecting the response of the material to at least one of neutron and gamma ray irradiation. Of course, other components as would be understood by the skilled artisan reading the present descriptions may be included and/or excluded according to various approaches.

In various embodiments, digital processing for pulse shape discrimination may be performed substantially as described in U.S. patent application Ser. No. 13/024,066, filed Feb. 9, 2011, which is incorporated in its entirety herein by reference.

Applications and Uses

Embodiments of the present invention may be used in a wide variety of applications, and potentially any application in which high light yield and/or pulse shape discrimination between gammas, neutrons, charged particles, etc. is useful.

Illustrative uses of various embodiments of the present invention include, but are not limited to, applications requiring radiation detection. Search, surveillance and monitoring of radioactive materials are a few such examples. Various embodiments can also be used in the nuclear fuel cycle, homeland security applications, nuclear non-proliferation, medical imaging, special nuclear material, high energy physics facilities, etc. Moreover, the figure of merit (FOM) performance metric is already sufficient to distinguish neutrons from gammas down to the few hundred keV/gamma equivalent regime, and will be very useful for non-proliferation, homeland security and safeguards applications.

Yet other uses include detectors for use in treaty inspections that can monitor the location of nuclear missile warheads in a nonintrusive manner. Further uses include implementation in detectors on buoys for customs agents at U.S. maritime ports, cargo interrogation systems, and instruments that emergency response personnel can use to detect or search for a clandestine nuclear device. Assessment of radiological dispersal devices is another application.

While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 

What is claimed is:
 1. A scintillator material comprising: a polymer matrix; and a primary dye in the polymer matrix, the primary dye being a fluorescent dye, the primary dye being present in an amount of 10% or more; wherein the primary dye comprises 2,5-diphenyl oxazole (PPO).
 2. The scintillator material of claim 1, wherein the primary dye is present in an amount of 30 wt % or more.
 3. The scintillator material of claim 1, wherein the primary dye is present in an amount ranging from about 30 wt % to about 75 wt %.
 4. The scintillator material of claim 1, wherein the polymer matrix comprises at least one polymer selected from the group consisting of: polyvinyl xylene (PVX); polyvinyl diphenyl; and polyvinyl tetrahydronaphthalene.
 5. The scintillator material of claim 1, wherein the scintillator material exhibits an optical response signature for neutrons that is different than an optical response signature for gamma rays.
 6. The scintillator material of claim 1, comprising a secondary dye, wherein the secondary dye has a longer emission wavelength than the primary dye; wherein the secondary dye comprises 1,4-bis(2-methylstyryl)benzene (Bis-MSB); and wherein the secondary dye is present in an amount of less than 2 wt % and more than 0.2 wt %.
 7. The scintillator material of claim 1, wherein the primary dye is crosslinked to the polymer matrix.
 8. The scintillator material of claim 1, wherein the scintillator material is formed into a rigid, solid plastic structure.
 9. The scintillator material of claim 1, wherein the primary dye includes multiple types of fluorescent dyes.
 10. A system comprising the scintillator material of claim 1, the system further comprising: a photodetector coupled to the scintillator material, the photodetector being configured to detect an optical response signature of the scintillator material to at least one of neutron and gamma ray irradiation; and a processor coupled to the photodetector, the processor being configured to process an output of the photodetector using pulse shape discrimination and differentiate responses of the scintillator material to the neutron and gamma ray irradiation.
 11. The scintillator material of claim 1, further comprising a secondary dye in the polymer matrix present in an amount from about 0.2 wt % to about 2 wt %; and wherein the secondary dye comprises a compound selected from the group consisting of: diphenyl anthracene (DPA), tetraphenyl butadiene (TPB), 1,4-Bis(5-phenyl-2-oxazolyl)benzene (POPOP) and 1,4-bis(2-methylstyryl)benzene (Bis-MSB).
 12. The scintillator material as recited in claim 11, further comprising: an initiator in the polymer matrix; wherein the secondary dye is characterized by an emission wavelength longer than an emission wavelength of the primary dye; wherein the primary dye is present in an amount ranging from about 50 wt % to about 75 wt %; wherein the primary dye is crosslinked to the polymer matrix; wherein the primary dye includes multiple types of fluorescent dyes; and wherein a total loading of dye in the scintillator material is sufficient to cause the scintillator material to exhibit a pulse-shape discrimination (PSD) figure of merit (FOM) of about at least 2.0.
 13. A scintillator material comprising: a polymer matrix comprising: polyvinyl xylene (PVX); polyvinyl diphenyl; and/or polyvinyl tetrahydronaphthalene; a primary dye in the polymer matrix, the primary dye being a fluorescent dye, the primary dye being present in an amount of greater than 10 wt %; and a secondary dye in the polymer matrix; wherein the primary dye comprises 2,5-diphenyl oxazole (PPO); and wherein the secondary dye comprises a compound selected from: diphenyl anthracene (DPA), tetraphenyl butadiene (TPB), 1,4-Bis(5-phenyl-2-oxazolyl)benzene (POPOP) and 1,4-bis(2-methylstyryl)benzene (Bis-MSB).
 14. The scintillator material of claim 13, further comprising an initiator in the polymer matrix.
 15. The scintillator material of claim 13, the secondary dye having a longer wavelength than the primary dye, wherein the secondary dye is present in an amount of less than 2 wt %; and wherein the scintillator material exhibits a pulse shape discrimination (PSD) figure of merit (FOM) of at least about 3.0.
 16. The scintillator material of claim 13, wherein the primary dye is characterized as being either crosslinked or copolymerized with another component of the polymer matrix.
 17. The scintillator material of claim 13, wherein the primary dye includes multiple types of fluorescent dyes.
 18. The scintillator material as recited in claim 13, wherein the polymer matrix consists of the: polyvinyl xylene (PVX); the polyvinyl diphenyl; and/or the polyvinyl tetrahydronaphthalene.
 19. The scintillator material as recited in claim 13, wherein the scintillator material exhibits an optical response signature for neutrons that is different than an optical response signature for gamma rays; and the scintillator material further comprising: a photodetector coupled to the scintillator material, the photodetector being configured to detect the optical response signature of the scintillator material to at least one of the neutrons and the gamma rays; and a processor coupled to the photodetector, the processor being configured to process an output of the photodetector using pulse shape discrimination and differentiate responses of the scintillator material to the neutron and gamma ray irradiation.
 20. The scintillator material as recited in claim 13, wherein a total loading of dye in the scintillator material is sufficient to cause the scintillator material to exhibit a pulse-shape discrimination (PSD) figure of merit (FOM) of about at least 2.0.
 21. A scintillator material comprising: a polymer matrix; and a primary dye in the polymer matrix, the primary dye being a fluorescent dye, the primary dye being present in an amount of 5 wt % or more; and a secondary dye in the polymer matrix; wherein the primary dye comprises 2,5-diphenyl oxazole (PPO); wherein the secondary dye comprises a compound selected from: diphenyl anthracene (DPA), tetraphenyl butadiene (TPB), 1,4-Bis(5-phenyl-2-oxazolyl)benzene (POPOP) and 1,4-bis(2-methylstyryl)benzene (Bis-MSB); and wherein the scintillator material exhibits an optical response signature for neutrons that is different than an optical response signature for gamma rays. 